Theses and Dissertations - Department of Aerospace Engineering and Mechanics

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Adaptive highly flexible multifunctional wings for active and passive control and energy harvesting with piezoelectric materials
(University of Alabama Libraries, 2017) Tsushima, Natsuki; Su, Weihua; University of Alabama Tuscaloosa
The purpose of this dissertation is to develop an analytical framework to analyze highly flexible multifunctional wings with integral active and passive control and energy harvesting using piezoelectric transduction. Such multifunctional wings can be designed to enhance aircraft flight performance, especially to support long-endurance flights and to be adaptive to various flight conditions. This work also demonstrates the feasibility of the concept of piezoelectric multifunctional wings for the concurrent active control and energy harvesting to improve the aeroelastic performance of high-altitude long-endurance unmanned air vehicles. Functions of flutter suppression, gust alleviation, energy generation, and energy storage are realized for the performance improvement. The multifunctional wings utilize active and passive piezoelectric effects for the efficient adaptive control and energy harvesting. An energy storage with thin-film lithium-ion battery cells is designed for harvested energy accumulation. Piezoelectric effects are included in a strain-based geometrically nonlinear beam formulation for the numerical studies. The resulting structural dynamic equations are coupled with a finite-state unsteady aerodynamic formulation, allowing for piezoelectric energy harvesting and active actuation with the nonlinear aeroelastic system. This development helps to provide an integral electro-aeroelastic solution of concurrent active piezoelectric control and energy harvesting for wing vibrations, with the consideration of the geometrical nonlinear effects of slender multifunctional wings. A multifunctional structure for active actuation is designed by introducing anisotropic piezoelectric laminates. Linear quadratic regulator and linear quadratic Gaussian controllers are implemented for the active control of wing vibrations including post-flutter limit-cycle oscillations and gust perturbation. An adaptive control algorithm for gust perturbation is then developed. In this research, the active piezoelectric actuation is applied as the primary approach for flutter suppression, with energy harvesting, as a secondary passive approach, concurrently working to provide an additional damping effect on the wing vibration. The multifunctional wing also generates extra energy from residual wing vibration. This research presents a comprehensive approach for an effective flutter suppression and gust alleviation of highly flexible piezoelectric wings, while allowing to harvest the residual vibration energy. Numerical results with the multifunctional wing concept show the potential to improve the aircraft performance from both aeroelastic stability and energy consumption aspects.
Additive manufacturing of two phase thermoplastic composites: a process model, microstructure and performance study
(University of Alabama Libraries, 2019) Papon, Md Easir Arafat; Haque, Anwarul; University of Alabama Tuscaloosa
Fused filament fabrication (FFF) based additive manufacturing (AM) of polymers and composites is a growing interest in processing tailorable parts with functional requirements like structural integrity, lightweight, high-temperature capability, etc. In general, the parts manufactured by FFF show large void contents, weak bonding, and inferior structural performance in comparison to those produced by conventional methods. The present research focused on overcoming the shortcomings of FFF through process modeling, microstructure study, and performance analysis. An experimental and numerical study has been conducted on the FFF of carbon fiber reinforced polylactic acid (CF/PLA) composites. A computational fluid dynamics (CFD) based numerical model was developed to simulate the temperature distribution and melt flow characteristics of highly viscous polymer (single and two-phase composites) using non-Newtonian computational model. Free space bead flow geometry and bead spreading architecture on the platform were also simulated with various nozzle geometries. The effects of the circular, square, and star-shaped geometries on bead cross-sectional shapes were studied both numerically and experimentally to achieve less void contents and improve the bead/layer bonding. Different dominant FFF process variables, both in filament extrusion and part production steps were studied, and a multi-level experimentation scheme was developed to study the bead-level to part-level properties. Physics-based surrogate models were developed, and stochastic uncertainty analysis was carried out on the manufacturing process to build up an optimum process-structure relationship. Another criticality of fiber-matrix interfacial bonding in the FFF-composites was addressed by introducing proper surface treatment to the fibers and post-manufacturing thermal treatment. The numerical model showed good promise in tailoring the bead geometry with the square and star-exit nozzle that potentially enhanced the bead to bead bonding. Extensive experimental studies were conducted to characterize strength, stiffness, fracture toughness, and void contents with various printed layer orientations and fiber concentrations of the FFF coupons. An acid-based functionalization of fibers, printing using square-nozzle, and enhanced crystallinity through controlled annealing were found to improve the fiber-matrix and inter-bead bonding, reduce the inter and intra-void and improve mechanical performances. The optimization and experimental data-driven stochastic modeling of the process parameters paved the way for producing parts with greater confidence at reduced experimental affords. The investigations and strategies developed in this dissertation will help to establish a high-quality and efficient process framework to improve the performance of additively manufactured two-phase composites. The fundamental understating and knowledge exercised in this dissertation can potentially be used for any polymer-based AM processes beyond the FFF since the fundamental challenges of controlling the voids and bonding are unavoidable.
Aerodynamic comparisons of membrane wings with cambered and flat frames at low reynolds number
(University of Alabama Libraries, 2016) Wrist, Andrew Harley; Hubner, James Paul; University of Alabama Tuscaloosa
The limited size of micro air vehicles (MAVs) requires small power sources, leading to a need for high aerodynamic efficiency. Flexible membrane wings at the MAV scale can experience improved lift/drag ratios, delays in stall, and decreased time-averaged flow separation when compared to rigid wings. This research thesis examines the effect of frame camber on the aerodynamic characteristics of membrane wings. The frames for the wings were designed in SolidWorks and constructed using an Objet30 Pro 3D printer. The membranes are composed of silicone rubber. Tests were conducted in The University of Alabama’s low-speed wind tunnel in 135 Hardaway Hall in low Reynolds number flow (Re ~ 50,000). Aerodynamic force and moment measurements were acquired at angles-of-attack varying from -4 to 24°. The results were used to determine whether cambered frames provide membrane wings with aerodynamic advantages when compared to those with flat frames. Additionally, a digital image correlation (DIC) camera system was used to acquire time-averaged shapes for the membrane wings during wind tunnel tests. The wings were mounted vertically at angles-of-attack of 6° and 18° to represent the regions of maximum efficiency and approaching stall, respectively. An in-house MATLAB program was developed to average the deflection plots from the images and produce time-averaged shapes. Lifting-line theory was applied to the time-averaged shapes to calculate theoretical lift and induced drag coefficients. The experimental set-up, results, and conclusions are discussed.
Analysis of beveled semi-elliptical surface cracks in friction stir plug welded plates made of Al 2195 alloy
(University of Alabama Libraries, 2011) Vadakke Veetil, Rahul; Barkey, Mark E.; University of Alabama Tuscaloosa
Friction Stir Welding (FSW) is a solid state joining process primarily used for Al alloys. Friction Stir Plug Welding (FPW) is a process in which a tapered shaped plug is friction stir welded into the hole that was left in the welded part when the initial FSW tool was removed. A rectangular plate made of Al 2195 alloy with a friction welded plug and containing a semi-elliptical surface crack was analyzed using the help of the software `FEA Crack'. Three different crack depths of deep cracks as well as shallow cracks were considered in identical plates of a quarter inch thickness. The depths were 0.08, 0.13 and 0.18 inches for deep cracks and 0.008, 0.013 and 0.018 inches for shallow cracks. A uniaxial tensile load of 1 psi was applied on one end surface with the opposite surface being fixed. For each depth, four different crack arc lengths were considered which were of 15, 30, 60 and 90 degrees. For each of these cases, the crack tube containing the crack was rotated around the plug having an inner bevel, in steps of 10 degree starting from the base position to the 90 degree (vertical) position with an additional case of 45 degree rotation in between. The stress intensity factor K was plotted against the crack front angle. The average and maximum K factor values were also plotted for each of the main crack lengths against the crack rotation angle. The same procedure was employed for shallow cracks. The results were validated using Newman-Raju equations for semi elliptical surface cracks. Non dimensional K factor plots were also made for different cases of both deep and shallow surface cracks. Researchers studying the surface cracks can now get an estimate of the value of stress intensity factor for the crack length, depth of crack and also the angular position of the crack around the plug by interpolating my results.
Application of direct method transcription for a human-class translunar injection trajectory optimization
(University of Alabama Libraries, 2011) Witzberger, Kevin Eric; Zeiler, Thomas A.; University of Alabama Tuscaloosa
This thesis presents a new trajectory optimization software package developed in the framework of a low-to-high fidelity three degree-of-freedom (3-DOF)/6-DOF vehicle simulation program named Mission Analysis Simulation Tool in Fortran (MASTIF) and its application to a translunar trajectory optimization problem. The functionality of the developed optimization package is implemented as a new ``mode" in generalized settings to make it applicable for a general trajectory optimization problem. In doing so, a direct optimization method using collocation is employed for solving the problem. Trajectory optimization problems in MASTIF are transcribed to a constrained nonlinear programming (NLP) problem and solved with SNOPT, a commercially available NLP solver. A detailed description of the optimization software developed is provided as well as the transcription specifics for the translunar injection (TLI) problem. This assessment of the final results is formulated via a metric given as the minimization of the TLI main engine burn time, which is equivalent to the maximization of the mass at main engine cutoff (MECO). Key design parameters include the initial values for three orbital angles (right ascension of ascending node, argument of perigee, and true anomaly) and three Euler angles for steering during the main engine burn. To do so, the solution starts by modeling the entire trajectory into three distinct phases. The first two phases are based on a collocation method whereas the third phase appears with a high order Runge-Kutta integration. The next part of assessing the TLI trajectory utilizes MASTIF's vehicle simulation capabilities (the other "mode" within MASTIF). This includes the ability to design and test new and existing guidance, navigation, and control (GN\&C) algorithms. As a demonstration of MASTIF's versatility, results from the trajectory optimization (the open-loop solution) in the form of a set of initial states and specific orbital target parameters at MECO are used in a new preliminary assessment of a variant of the Space Shuttle's flight-proven closed-loop guidance algorithm named Powered Explicit Guidance (PEG). Main engine burn times and the LVLH Euler angles from the open-loop and closed-loop solutions are compared to show approximate agreement and efficacy of MASTIF's two distinct "modes''.
Application of Numerical Methods to the Hamilton-Jacobi-Isaacs Equation in Various Dynamical Systems
(University of Alabama Libraries, 2021) Ledbetter, William Gordon; Sood, Rohan; University of Alabama Tuscaloosa
The field of differential games has broad applicability to topics of economics, engineering, business, and warfare. Given the increasing levels of autonomy implemented in man-made systems in these fields, competition-based analysis may be the best option for understanding behavioral bounds when such systems interact. Differential games are governed by the Hamilton-Jacobi-Isaacs PDE, and many solution techniques are explored before identifying a gap in the existing literature. This dissertation develops a new approach to analyzing differential games based on a saddle-point solution technique. In a 2D system, the standard algorithmic approach produces both a value function interpolation and an approximate control map. Additionally, analysis of the observation error indicates that future analysis should prefer a problem formulation with relative motion. In the Circular Restricted Three-Body Problem, the same algorithms are applied to a system with real-world implications. The value and control interpolations produce a near-optimal trajectory, but the radial basis function approach suffered from high data density and did not exactly recreate the nominal solution. A perturbation analysis indicated that any mid-flight disturbance to the game state is most likely to benefit the pursuer, extending the works of Isaacs to a new domain. Ultimately, the proposed method is demonstrated to be a valuable tool for future differential games research.
Application of shark skin flow control techniques to airflow
(University of Alabama Libraries, 2017) Morris, Jackson Alexander; Lang, Amy W.; Hubner, James Paul; University of Alabama Tuscaloosa
Due to millions of years of evolution, sharks have evolved to become quick and efficient ocean apex predators. Shark skin is made up of millions of microscopic scales, or denticles, that are approximately 0.2 mm in size. Scales located on the shark’s body where separation control is paramount (such as behind the gills or the trailing edge of the pectoral fin) are capable of bristling. These scales are hypothesized to act as a flow control mechanism capable of being passively actuated by reversed flow. It is believed that shark scales are strategically sized to interact with the lower 5% of a boundary layer, where reversed flow occurs at the onset of boundary layer separation. Previous research has shown shark skin to be capable of controlling separation in water. This thesis aims to investigate the same passive flow control techniques in air. To investigate this phenomenon, several sets of microflaps were designed and manufactured with a 3D printer. The microflaps were designed in both 2D (rectangular) and 3D (mirroring shark scale geometry) variants. These microflaps were placed in a low-speed wind tunnel in the lower 5% of the boundary layer. Solid fences and a flat plate diffuser with suction were placed in the tunnel to create different separated flow regions. A hot film probe was used to measure velocity magnitude in the streamwise plane of the separated regions. The results showed that low-speed airflow is capable of bristling objects in the boundary layer. When placed in a region of reverse flow, the microflaps were passively actuated. Microflaps fluctuated between bristled and flat states in reverse flow regions located close to the reattachment zone.
Atomistic modeling and structure-property relationship of topologically accurate complex nanotube junction architectures
(University of Alabama Libraries, 2020-08) Nakarmi, Sushan; Barkey, Mark; Unnikrishnan, Vinu; University of Alabama Tuscaloosa
Carbon nanotubes have remarkable material properties and are ideal for different space applications including thermal management devices, light-weight mechanical shock absorbers, and fiber-reinforced composites. Nanotube junctions, which are the interconnections of carbon nanotubes, have properties different from the pristine structures and are promising materials for constructing unit blocks with excellent material properties. However, widespread application of the junctions and nanostructures is limited due to the lack of understanding of their mechanical, thermal, and electronic properties. The overall objective of the current research is to provide a computational methodology to construct atomistic models of nanostructures and study their thermal and mechanical properties under different operating conditions. In the first part of the research, the topologically accurate atomistic models of the junctions are created using a novel CAD-based remeshing and optimization strategies. The most energetically stable configurations are chosen to build 3D architectures, thus, providing an economical way to construct complex and larger dimension nanostructures. The created macro-structures can be used directly in the atomistic simulations to study their structure-property relationships. In this dissertation, the thermal and mechanical characterization of pristine nanotubes and complex nanotube multiterminal junctions have been studied using molecular dynamics (MD) simulation. At the nanoscale, the thermal conductivity of nanotube is found to be dependent on size, strain, temperature, and defects. The effects of each of these parameters on the thermal transport of nanostructures have been determined using MD. This is followed by the comparative study of the phonon density of states and phonon dispersion relations of different configurations. The study provides guidelines for creating nanotube heat transfer devices with desired thermal specifications. In addition to being highly conductive, nanotubes and junctions have very high strength and modulus. Although an extensive amount of research is available with the characterization of the pristine nanotubes, there lacks a proper understanding of the mechanical characteristics of the complex structures (multi-terminal junctions and micro-blocks). With the atomistic models of these structures created, the tensile and compressive strengths of such complex architectures have been presented. These computational models will provide the much needed next step for the realization of nanotube junctions for the industrial applications.
Biofidelic soft composites– experimental and computational modeling
(University of Alabama Libraries, 2017) Chanda, Arnab; Unnikrishnan, Vinu U.; University of Alabama Tuscaloosa
Biofidelic soft composites or tissues form the building blocks of the human body. Understanding the complex mechanics of these soft composites is the key to understanding the genesis and progression of disease. Biomechanically, soft composites exhibit anisotropic mechanical behavior and comprise of multiple fiber layers within a soft matrix. To date, there is a lack of understanding of the anisotropic mechanical behavior of soft composites, primarily due to unavailability of a robust characterization framework. In this dissertation, novel multiscale computational and experimental investigation models are developed to simulate and characterize anisotropic soft composite mechanical behavior. Soft composite surrogates were first developed to simulate various tissues in the human body namely the skin, brain, artery and vaginal tissues. Novel anisotropic soft composite models were also fabricated taking into consideration the tissue anisotropy and multifunctional properties. Hyperelastic anisotropic constitutive relationships were formulated to precisely characterize the mechanical behavior of soft composite considering varying fiber and matrix contributions, fiber-matrix interactions, fiber orientations and multiple fiber layers. Coupled with high fidelity experimental and computational models, microscopy, and Digital Image Correlation (DIC) studies, the damage and repair of soft composite surrogates are also discussed in this dissertation, with relevance to soft tissue wounds and suture. Computational modeling to understand the interaction between multiple soft composite systems and its effect on soft composite damage are also highlighted in this work. Some specific soft composite interaction systems modeled were the female pelvic system under abdominal loads, whole body impact due to blast, and ulceration in diabetic foot. This dissertation lays the foundation for micro and macro scale anisotropic soft composite modeling and characterization using high fidelity experimental and numerical techniques which will be indispensable for studying tissue mechanics and other soft composite applications in engineering and medicine.
Borescopic Laser Doppler Velocimetry probe
(University of Alabama Libraries, 2013) O'Brien, Kory Thomas; Olcmen, Semih M.; University of Alabama Tuscaloosa
A miniature fiber-optic, single-velocity-component Laser Doppler Velocimetry (LDV) probe for measurement in cramped spaces, where access is very limited, has been designed, constructed, and tested. The probe design allows the main probe dimensions to be small (7mm in diameter). In addition, the proposed back-scatter collection scheme allows the main section to be as long as needed to access remote locations. The laser beams are first collimated by passing them through two separate collimating lenses. The collimated light then passes through 1 mm holes machined into a right angle prism-mirror and are focused to form the measurement probe volume using the focusing lens placed at the end of the probe extension tube. The light scattered by the particles in the flow is collected back by the focusing lens and is collimated. The collimated light then reflects off the right-angle mirror by 90 degrees, passes through the receiving lens, and is focused to the receiving fiber terminator. The receiving fiber-optic cable transmits the collected light to the photo-multiplier tube which then converts the signal into an electrical signal for further processing of the data. The probe working principle was proven in two types of jet flows.
Carbon nanotube sheet scrolled fiber composite for enhanced interfacial mechanical properties
(University of Alabama Libraries, 2017) Kokkada Ravindranath, Pruthul; Roy, Samit; University of Alabama Tuscaloosa
The high tensile strength of Polymer Matrix Composites (PMC) is derived from the high tensile strength of the embedded carbon fibers. However, their compressive strength is significantly lower than their tensile strength, as they tend to fail through micro-buckling, under compressive loading. Fiber misalignment and the presence of voids created during the manufacturing processes, add to the further reduction in the compressive strength of the composites. Hence, there is more scope for improvement. Since, the matrix is primarily responsible for the shear load transfer and dictating the critical buckling load of the fibers by constraining the fibers from buckling, to improve the interfacial mechanical properties of the composite, it is important to modify the polymer matrix, fibers and/or the interface. In this dissertation, a novel approach to enhance the polymer matrix-fiber interface region has been discussed. This approach involves spiral wrapping carbon nanotube (CNT) sheet around individual carbon fiber or fiber tow, at room temperature at a prescribed wrapping angle (bias angle), and then embed the scrolled fiber in a resin matrix. The polymer infiltrates into the nanopores of the multilayer CNT sheet to form CNT/polymer nanocomposite surrounding fiber, and due to the mechanical interlocking, provides reinforcement to the interface region between fiber and polymer matrix. This method of nano-fabrication has the potential to improve the mechanical properties of the fiber-matrix interphase, without degrading the fiber properties. The effect of introducing Multi-Walled Carbon Nanotubes (MWNT) in the polymer matrix was studied by analyzing the atomistic model of the epoxy (EPON-862) and the embedded MWNTs. A multi-scale method was utilized by using molecular dynamics (MD) simulations on the nanoscale model of the epoxy with and without the MWNTs to calculate compressive strength of the composite and predict the enhancement in the composite material. The influence of the bias/over wrapping angle of the MWNT sheets on the carbon fiber was also studied. The predicted compressive strength from the MD results and the multiscale approach for baseline Epoxy case was shown to be in good relation with the experimental results for Epon-862. On adding MWNTs to the epoxy system, a significant improvement in the compressive strength of the PMC was observed. Further, the effect of bias angle of MWNT wrapped over carbon fiber was compared for 0, 45 and 90. This is further verified by making use of the Halpin-Tsai.
Characterization and Modeling of Dual Phase Thermoplastic Self Healing System for Fiber Reinforced Thermoset Composite Structures
(University of Alabama Libraries, 2020) Jony, Bodiuzzaman; Roy, Samit; University of Alabama Tuscaloosa
The one characteristic that sets biological systems apart from human-engineered systems is its ability to heal itself repeatedly without any external intervention. In the last few decades, drawing inspiration from nature, there has been a tenacious drive towards the design and development of bio-mimetic multifunctional polymers and polymer matrix composites that possess the capability of repeatable self-healing. In this dissertation, bio-mimetic self-healing methods are explored for recover-ing the mechanical and structural performances of damaged ?ber reinforced thermoset polymer composite using thermoplastic healants. Speci?cally, repeatable Mode-I interlaminar fracture healing capabilities of thermoplastic polycaprolactone (PCL) particles and polyurethane shape memory polymer (SMP) ?brils in a thermoset unidirectional carbon/epoxy composite were investigated. During the bio-mimetic healing process, the polyurethane SMP ?brils were used to close the open crack through a thermally-activated contraction, and then the thermoplastic PCL was heated to heal the damage through melt intercalation into the crack. The chemical and thermal properties of the polymer composite and healants and interactions between the brittle epoxy and ductile healants were investigated. Further, repeatable Mode-II interlaminar shear fracture property recovery of unidirectional carbon/epoxy by a blend of the same biphasic healants was experimentally investigated. The shear crack growth phenomenon and mechanism of fracture toughness recovery were thoroughly investigated and contrasted with Mode-I failure. Finally, the real-time in-situ application of self-healing in ?ber-reinforced composite was accomplished by using a macro ?ber composite (MFC) actuator assisted healing. The parameters for generating stimulus (heat) from MFC without damaging it or the composite were calculated. Relative crack growth stability was also investigated during in-situ healing for virgin and healed cases by using R ? curve and crack growth rate phenomenon. For a comprehensive understanding of the healing mechanism and fracture behavior of the polymer composite, analytical and numerical models were generated using a bilinear cohesive law. The critical fracture parameters obtained from the analytical studies were thoroughly veri?ed with experimental results and ?nite element numerical simulations. It is envisioned that this work will provide a solid foundation for the future development and implementation of self-healing polymer composites in real life structural applications.
Characterization and modeling of the effect of environmental degradation on flexural strength of carbon/epoxy composites
(University of Alabama Libraries, 2010) Chawla, Sneha Anil; Roy, Samit; University of Alabama Tuscaloosa
A mechanism-based modeling methodology has developed for prediction of long-term durability of composites for emerging facilities in different climatic zones. The objective of the research was to develop a predictive tool using the Arrhenius principles adapted to the TTS (Time Temperature Superposition) to measure degradation of carbon-fiber/epoxy composite under hygrothermal exposure and applied tensile stress. The hygrothermal conditions capture the synergistic effects of field exposure and extreme temperatures, viz., hot/dry, hot/wet, cold/dry, and cold/wet. Short term tests were performed to determine the flexural strength of environmentally aged composite specimens in accordance with ASTM D2344-84 and ASTM D7264 respectively. Carbon/epoxy specimens of [02/902]2s configuration were manufactured for flexure tests using Vacuum Assisted Resin Transfer Molding (VARTM). A unique strain fixture was designed to apply constant strain on the specimens during ageing and applied a simple methodology to eliminate excessive creep in the specimens. A two-dimensional cohesive layer constitutive model with a cubic traction-separation law has being developed in order to predict the life of the composite under hygrothermal conditions. The model simulated the test conditions and predicted the progressive failure mechanism of the specimen as observed in the tests, under various loading conditions. The model also incorporated synergistic interactions between temperature, moisture and stress effects and predicted degradation in strength and stiffness as a function of different ageing conditions and ageing times. Model predictions have been benchmarked using test data.
Computational analysis of diffuser performance for the Subsonic Aerodynamic Research Laboratory wind tunnel
(University of Alabama Libraries, 2012) King, Christopher David; Olcmen, Semih M.; University of Alabama Tuscaloosa
The Air Force has expressed interest in improving the efficiency of the Subsonic Aerodynamic Research Laboratory (SARL) wind tunnel. In a previous analysis of losses throughout the tunnel, it was found that approximately thirty percent of pressure losses through the tunnel occurred at the exit of the tunnel (Britcher, 2011). The use of alternative diffuser geometries in reducing pressure losses at the exit of the tunnel and the computation of their efficiency improvement with respect to the original tunnel geometry and with respect to each other for the SARL wind tunnel are the focus of this research. Three different diffuser geometries were evaluated numerically using both the SolidWorks Flow Simulation add-on, and ANSYS FLUENT. For each of these geometries, a scaled down model was manufactured to be used for experimental validation in future work. Both the full size and small scale numerical models were evaluated with an inlet velocity of sixty meters per second. As the nature of the flow at this point in the wind tunnel is not known, both a uniform and fully developed turbulent flow profiles were evaluated for each design, both for the small scale models and the full size models, to determine pressure losses with respect to the varying flow types entering the diffusers. This research seeks to determine the effects of these different geometries on the flow downstream of the exit, and the possible energy savings associated with each design. In addition, it seeks to compare the numerical results obtained from both SolidWorks Flow Simulation and ANSYS FLUENT.
Concurrently coupled multiscale modeling of polymer nanocomposites
(University of Alabama Libraries, 2016) Li, Shibo; Roy, Samit; University of Alabama Tuscaloosa
Embedded statistical coupling method (ESCM) was originally developed to provide computational efficiency, to decrease coupling complexities, and to avoid the need to discretize the continuum model to atomic scale resolution in concurrent multiscale modeling. ESCM scheme is relatively easy to implement within conventional FEM code and has been tested in standard solid lattice structures. However, this method encounters difficulties when being implemented for amorphous materials like polymers, due to the fact that they lack specific ordered lattice structure and atoms may not be covalently bonded with each other, which are the requirements of common coupling schemes. Therefore, a new approach needs to be developed to resolve this problem. In this paper, details of a modified ESCM approach for atomistic-continuum coupling developed to perform simulations of crack growth in polymers is presented. The presence of the continuum domain surrounding the MD region allows for the application of far-field loading, and prevents stress wave reflections from the external boundary impinging back on the crack tip. In our approach, Material Point Method (MPM), which is a meshless particle-in-cell method based on Arbitrary Euler-Lagrange (ALE) scheme and has been proven to have good performance in large deformation problems, is used to model the continuum domain. It is concurrently coupled with molecular dynamics (MD), a widely used method in atomistic simulations, using a so-called handshake region. Anchor points, the equilibrium positions of the constrained particles, which are designed to transmit displacements and forces between nanoscale and macroscale model, are defined in the handshake region. A concurrently coupled MPM-MD simulation of crack propagation inside a polymer is performed to verify this new coupling approach, thereby providing a better understanding of the fracture mechanisms at the nanoscale to predict the macro-scale fracture toughness of polymer system. Results are presented for concurrently coupled crack propagation simulation in a di-functional cross-linked thermoset polymer, EPON 862. The composite laminate open hole tension problem is also studied using concurrent multiscale approach by implementing micromechanics program MAC/GMC in FEA frame.
Construction of a multi-functional cryogenic propellant tank with cross-linked silica aerogel
(University of Alabama Libraries, 2010) Reinheimer, Preston Glenn; Roy, Samit; University of Alabama Tuscaloosa
Aerogels are low-density nanostructured porous materials, whose practical applications have been limited by their poor mechanical properties. Crosslinking the nanoparticle building blocks of silica aerogels with polymeric tethers increases both the modulus and the strength significantly. The polymer coating preserves the mesoporous structure of the silica framework while retaining its low thermal conductivity. The uniqueness of crosslinked silica aerogel has load carrying capabilities in which are determined in tensile, compression and flexural bending tests. Crosslinked silica aerogel testing displays specific compressive strength of 389000 Nm/Kg. Ballistic testing of crosslinked silica aerogel also corroborates its mechanical properties displaying a ballistic limit up to 80 m/s. Its thermal conductivity at 0.041 W/mK supports the use of crosslinked silica aerogel in cryogenic fuel cell applications. Manufacturing practices have been evaluated to obtain an optimal process which reduces time, money and difficulty.
Control surface hinge moment prediction using computational fluid dynamics
(University of Alabama Libraries, 2016) Simpson, Christopher David; O'Neill, Charles R.; University of Alabama Tuscaloosa
The following research determines the feasibility of predicting control surface hinge mo- ments using various computational methods. A detailed analysis is conducted using a 2D GA(W)-1 airfoil with a 20% plain flap. Simple hinge moment prediction methods are tested, including empirical Datcom relations and XFOIL. Steady-state and time-accurate turbulent, viscous, Navier-Stokes solutions are computed using Fun3D. Hinge moment coefficients are computed. Mesh construction techniques are discussed. An adjoint-based mesh adaptation case is also evaluated. An NACA 0012 45-degree swept horizontal stabilizer with a 25% ele- vator is also evaluated using Fun3D. Results are compared with experimental wind-tunnel data obtained from references. Finally, the costs of various solution methods are estimated. Results indicate that while a steady-state Navier-Stokes solution can accurately predict control surface hinge moments for small angles of attack and deflection angles, a time- accurate solution is necessary to accurately predict hinge moments in the presence of flow separation. The ability to capture the unsteady vortex shedding behavior present in mod- erate to large control surface deflections is found to be critical to hinge moment prediction accuracy. Adjoint-based mesh adaptation is shown to give hinge moment predictions similar to a globally-refined mesh for a steady-state 2D simulation.
Damage detection and sensor placement optimization in composite structures
(University of Alabama Libraries, 2012) Zheng, Hao; Roy, Samit; University of Alabama Tuscaloosa
Structural health monitoring, damage identification method and sensor placement optimization for carbon fiber reinforced polymer (CFRP) composite beam were studied in this thesis. In this work, different methodologies were investigated for the damage detection process to enhance the use of current structural health monitoring systems by identifying the optimal sensor placement. Carbon fiber reinforced polymer composite materials were fabricated and the fabrication process based on vacuum assisted resin transfer molding (VARTM) is briefly introduced. Numerical analysis using finite element method was subsequently performed for a composite beam based on the material properties determined by performing experimental material characterization tests. Three benchmarking tests with different types of elements were performed to verify the best method for modeling the composite panel. Moreover, shear lag analysis was also presented to model an embedded crack in the composite panel which would be used in damage detection and optimization process. Based on the finite element analysis and static strain data extracted, a comparative study on two damage detection algorithms based on artificial neural network (ANN) and support vector machine (SVM) is presented. The viability of these two methods was demonstrated by analysis of the numerical model of composite beam with a crack embedded in it and the performance for each algorithm is also presented with different number of sensors and different noise levels. Two experiments are presented for the performance evaluation of damage detection and identification. To identify the optimal locations of sensors in the optimization process, a statistical probability based method using the combination of artificial neural networks and evolutionary strategy were developed to increase the detection rate of damage in a structure. The proposed method was able to efficiently increase the detection accuracy compared with a uniform distribution of sensors for a composite beam that was damaged in different locations. The finite element model of the composite coupon was used as a representation of the real structure. Static strain data from finite element simulation was extracted with different damage scenarios and used as feature vector for the classification process. Based on the performance of the classification for a given sensor configuration, updated sensor locations would be selected by changing the coordinates of these sensor locations using strategy parameters. The viability of this method was demonstrated by conducting different examples and significant number of simulations was performed to check the repeatability of the algorithm.
Damage evolution in composite materials under environmental ageing: a stochastic model with experimental validation
(University of Alabama Libraries, 2009) Rahman, Rezwanur; Haque, Anwarul; University of Alabama Tuscaloosa
This work emphasizes on predicting probability density function of damages or "number density of damage" in graphite/epoxy polymer matrix composite materials (PMC) under hygrothermal aging condition. A coupled Forward-Backward Stochastic Differential Equation (FBSDE) is proposed as a mathematical model to predict number density of damages. The FBSDE consists of damage nucleation and annihilation rate in terms of Brownian motion. The uncertainty in damage nucleation and annihilation rate is noticed by proposing these two terms as "Brownian motion with drift". In order to verify the proposed model, a quantitative analysis was carried out on a limited number of graphite/epoxy specimens manufactured by VARTM process. The specimens were kept in a hygrothermal condition with room temperature cycling. A rigorous quantitative analysis of damages was done by optical microscopic inspection at different stages of aging period. The damages were classified based on the criteria of their size. Finally, the experimentally collected data for number density of damages were verified by using the proposed FBSDE. A detail parametric study was carried out using FBSDE and best possible predicted data was validated with the experimental observation. A reasonable estimation was observed from the model output.
Design and analysis of a carbon composite flap for the Cirrus SF50 jet aircraft
(University of Alabama Libraries, 2017) Haefner, Andrew; Mulani, Sameer B.; University of Alabama Tuscaloosa
A new carbon fiber composite flap was designed and analyzed for the Cirrus SF50. This new flap will replace the existing aluminum flap and has the potential to save 5.30 lbs per aircraft. The new flap has the same OML profile as the existing flap and the same hinge locations. This allows the new flap to be either an upgrade option for customers or a supplemental type certificate (STC) option for aircraft in the field. The flap was designed with the same spar location and similar rib locations which allow existing tooling to be used for assembly. The design was analyzed to the static and damage tolerance requirements specified in 14 CFR Part 23. The loads that were utilized for the analysis were calculated using the method in 14 CFR Part 23 Appendix A. The loads are conservative since they consider a load factor of 3.6 instead of 2.2, this was done to make the design and analysis future proof. Since a significant portion of the structure uses minimum gauge layups (2 core 3, 4 ply solid), the weight increase from using the significantly higher load factor is minimal. The flap design and analysis are considered future proof because the loads used will be greater than the required loads if the SF50 were to have either a gross weight increase, a deployment speed increase, a deployment angle increase, or all in combination.

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